1. Technical Field
This invention relates to technology for determining the extent of blur in an image of an image data file.
2. Related Art
Technologies for selecting an in-focus image from among a number of photographic images have existed for some time. For example, in one conventional technology, during shooting a photograph the image pickup device is moved from a reference position to two locations forward and rearward, and an evaluation function is calculated for images taken at the respective locations. The reference position of the image pickup device is then changed either forward and rearward, whichever gives the better value for the evaluation function. This process is repeated to determine a focal point for the image pickup device.
The evaluation function is computed as follows. Specifically, frequency conversion is carried out for blocks of 8×8 pixels in the image, and 8×8 frequency components ak1 (k=1-8, l=1-8) are derived for each. The sum for all of the blocks of “the value derived by dividing the sum of the absolute values of the high frequency components (k+l>9) among the 8×8 frequency components ak1 (k=1-8, l=1-8), by the sum of the absolute values of the low frequency components (k+l<7)” is employed as the evaluation function.
In another conventional technology, a process such as the following is carried out when making the determination as to whether an image is blurred. Specifically, the square root of sum of squares g(x, y) is calculated for brightness differential fx, fy between a target pixel in an image and pixels neighboring the target pixel in the X direction and Y direction. Next, the average value of g(x, y) for all pixels in the image is computed by way of a “sharpness level SL.” The determination as to whether an image is blurred is made based on the magnitude of this SL.
However, with the former technology, it is necessary to compute a total of 48 components, i.e. the 28 components constituting the high frequency components (k+l>9) among the 8×8 frequency components ak1 (k=1-8, l=1-8) and the 21 components constituting the low frequency components (k+l<7), for all of the blocks in the image. That is, the volume of computations is enormous, and processing overhead is high. Moreover, this technology is adapted for comparing images of identical composition.
The latter technology, on the other hand, cannot be applied directly to images in the JPEG format, which saves an image in the form of coefficients of discrete frequency components, rather than a format that saves an image in the form of color information for the pixels, such as the BMP format. That is, where image data is data in the JPEG format, the entire image must first be converted to a format having color information for each pixel (e.g. the BMP format). Consequently, processing overhead is quite high.
With the foregoing in view, an aspect of the invention is directed to image data files containing images saved in the form of coefficients of discrete frequency components and is adapted to determine, with a minimum of processing overhead, the level of blurring of images of such image data files.
The entire disclosure of Japanese patent application No. 2007-9208, of SEIKO EPSON is hereby incorporated by reference into this document.